In a case where a short fault has occurred in one of an upper arm transistor and a lower arm transistor, which constitute a bridge circuit, when the other of the upper arm transistor and the lower arm transistor, which has no fault, is turned on, a short circuit current flows in these transistors, resulting in a secondary fault. A similar issue arises also in a case where an output terminal to be connected to a load is short-circuit to a power source line or in a case where a winding of a motor as a load has a short fault.
For example, the transistor is an insulated gate bipolar transistor (IGBT). In this case, when an on command is applied to a gate drive circuit, a gate voltage that is slightly higher than a threshold voltage is applied to a gate of the IGBT to operate the IGBT in an active region. A short fault can be detected based on a current detected when the IGBT is operated in the active region. When it is determined that there is no short fault, the gate voltage having a sufficient value is applied to the gate of the IGBT to operate the IGBT in a saturated region. Such a technology is, for example, disclosed in JP 2009-071956 A, which corresponds to US 2009/0066402 A1.
In such a structure, when the gate voltage is excessively high and exceeds a gate withstand voltage VGES, the IGBT is likely to be broken or the life time of the IGBT is likely to be shortened. In a conventional gate drive circuit, therefore, a clamp circuit is connected to the gate of the IGBT to protect the IGBT from a transitional increase of the gate voltage. The clamp circuit clamps (limits) the gate voltage to a predetermined voltage.
To protect the IGBT from an overcurrent condition due to a short fault or the like, that is, to perform a short circuit protection, it is necessary to consider the following points.
When the IGBT is shut off from the overcurrent condition by reducing the current, a surge (S) occurs according to an L component (inductance) of a current path through which a short circuit current flows and a change ratio (di/dt) of the short circuit current (i.e., S=−L×(di/dt)). When the surge exceeds the withstand voltage of the IGBT, there is a possibility that the IGBT is broken. Therefore, when the IGBT is shut off from the overcurrent condition by reducing the current, it is necessary to shut off the IGBT so that the short circuit current does not drastically change, and the surge does not exceed the withstand voltage of the IGBT.
In an abnormal time, such as in the above-described short fault condition, a collector voltage of the IGBT changes. Therefore, electric charge is injected to a gate capacitance through a parasitic capacitance (mirror capacitance) between the collector and the gate of the IGBT. That is, a mirror current flows. As a result, the gate voltage rises. With this, an on state of the IGBT is further deepened, that is, an on-state resistance of the IGBT is further lowered, resulting in an increase of the collector current (short circuit current). Therefore, a path for drawing the mirror circuit through the clamp circuit or the like is required.
However, the path of the short circuit current is different depending on the type of fault, as well as the L component will be different depending on the path. Therefore, the change ratio of the short circuit current is different depending on the type of fault, and the time required to determine whether the short circuit current is in an overcurrent condition is different.
Therefore, in the above-described technology, the following drawbacks will arise. For example, in a case where the L component of the current path is relatively large, even if the short fault has occurred, it is determined as a normal condition since the short circuit current does not reach the overcurrent after a predetermined period of time elapses. Thereafter, when the limiting of the gate voltage is cancelled and the sufficiently high gate voltage is applied to the gate, the current reaches the overcurrent. In this case, since there is no path for drawing the mirror current, the gate voltage further rises. Further, the short circuit current increases, and the shutting off of the IGBT is delayed. Moreover, the IGBT will be broken.
When the overcurrent has been determined in the state where the gate voltage is high, and when the gate voltage is drastically reduced so as to solve the overcurrent state, the collector current of the IGBT is rapidly limited. In this case, the collector current is reduced at a very high change ratio and a high surge voltage is generated. To solve this issue, it is considered to operate the clamp circuit all of the time in an on period of the IGBT. In this case, however, a period of time to flow the current through the clamp circuit is unnecessarily long, resulting in an increase in consumption current.